A typical Doherty amplifier has a main (carrier) amplifier biased to operate in a linear mode such as Class AB and a peaking amplifier biased to operate in a non-linear mode such as Class C. The signal input to the Doherty amplifier is split to each amplifier, and the amplified signals are recombined with an output combining network. Both amplifiers are operational when the input signal peaks, and are each presented with a load impedance that enables maximum power output. As the input signal decreases in power, the peaking amplifier turns off and only the main amplifier operates. At these lower power levels, the main amplifier is presented with a modulated load impedance that enables higher efficiency and gain. This results in an efficient solution for amplifying complex modulation schemes employed in current and emerging wireless systems e.g. such as WCDMA (Wideband CDMA), CDMA2000, and systems employing Orthogonal Frequency Division Multiplexing (OFDM), such as WiMAX (Worldwide Interoperability for Microwave Access) and the Long-Term Evolution (LTE) enhancement to the UMTS (Universal Mobile Telecommunications System) standard.
Traditional Doherty amplifier architectures are limited in radio frequency bandwidth because frequency dependant elements such as ¼ wavelength transmission lines are used in the output combining network. Accordingly, Doherty amplifiers are typically designed to operate within a specific narrow frequency band. For example, within the telecommunication field, the operating bandwidth is typically between 1-5% of the RF signal frequency. However, from a carrier perspective, a multiple band amplifier has significant cost benefits. Multiple band amplifiers can not be realized with a traditional Doherty implementation because of the frequency-limited elements present in the output combining network. For example, the Doherty combiner VSWR (voltage standing wave ratio) in the Zmod condition changes in response to small changes in the operating frequency band. As such, several traditional Doherty amplifiers must be used for wideband applications, with each amplifier designed for a specific narrow band within the wideband frequency range.